Syringe sharing behaviors among IDUs are typically based on self-reports and subject to socially desirable responding. We used three short tandem repeat (STR) genetic biomarkers to detect sharing in 2512 syringes exchanged by 315 IDUs in the Baltimore needle exchange program (738 person-visits). Demographic characteristics, direct and indirect needle sharing behaviors corresponding to the closest AIDS link to the intravenous experience (ALIVE) study visits were examined for association with multi-person-use of syringes. Overall, 56% of the syringes exchanged at Baltimore needle exchange program (NEP) had evidence of multi-person-use of syringes. Less multi-person-use of syringes (48 vs. 71%, p less than 0.0001) was seen with more rapid syringe turnaround (less than 3 days). IDUs always exchanging their own syringes ("primary" syringes) were less likely to return syringes with evidence of multi-person-use (52%) than those who exchanged syringes for others ("secondary" syringes); (64%, p=.0001), and those exchanging both primary and secondary syringes (58%, p=.004). In a multivariate analysis restricted to primary exchangers, multi-person-use of syringes was associated with sharing cotton (AOR=2.06(1.30-3.28)), lending syringes (AOR=1.70(1.24-2.34)) and injecting less than daily (AOR=0.64(0.43-0.95)). These findings support additional public health interventions such as expanded syringe access to prevent HIV and other blood-borne infections. Testing of STRs represents a promising approach to examining and accessing complex behavioral data including syringe sharing.